This invention relates to a method and apparatus for directly heat treating articles in a treatment chamber of a heat treatment furnace.
It is known in the art to directly heat treat metal articles in the treatment chamber of a heat treatment furnace. The term "to directly treat heat" refers to direct transfer of heat from the heat source to the articles to be heat treated in a controlled furnace atmosphere, i.e. direct transfer of heat from the products of combustion from a burner to the articles to be treated in a controlled atmosphere.
U.S. Pat. No. 2,763,476, Two Stage Combustion Furnace, by H. J. Ness et al, describes a furnace in which the products of combustion from burners positioned to fire directly into the furnace chamber are used to treat articles within the chamber. This patent describes an apparatus and method for directly heating work to high temperatures in the products of combustion of the heating fuel without scaling. Fuel and air are reacted in the heating chamber of the furnace in such a manner as to obtain a predetermined ratio of the constituents of the gaseous reaction products which is protective in nature to the work to be heated and at the same time maintaining a high temperature and heating rate in the furnace. A carbon dioxide to carbon monoxide ratio is obtained which is below the oxidizing ratio of these gases at the operating temperature in the furnace. The reaction products, in the Ness furnace, will have a high hydrogen and incompletely consumed carbon and hydrocarbon content.
In the Ness patent the roof of the furnace chamber has a plurality of spaced arched sections forming narrow channels. The sidewall of the furnace chambers has a series of burners in a position to fire into the channels and against the face of the arch. After-burned exhaust gases from the furnace chamber serve to heat the arched roof. The heating of the arched roof with the exhaust gases serves as an additional heat input into the work chamber. The arches and walls between the plurality of arches are heated to a temperature above the reaction temperature of the mixture supplied to the burners. This feature is important in this invention since it facilitates the reaction in the work chamber and permits air/fuel mixtures to be employed which include the endothermic range. These reactions are further facilitated by the channeled arrangement of the arch members whereby severe scrubbing of the entering furnace mixture occurs on the hot surfaces of the arch and walls between the arches. The catalytic effect of the hot brick work resulting from the scrubbing referred to above promotes the completion of these reaction,s whether they be endothermic or exothermic.
The Ness patent further discloses that in some cases it may be preferable to supply a air/fuel mixture to the burners which will produce a carbon monoxide content somewhat lower than what is desired at the work, and to increase this content by the subsequent addition of raw gas to the furnace at a point where the primary reactions have been completed. For this purpose the furnace is provided with a number of gas addition tubes entering the heating chamber below the arch. The tubes can be supplied with any suitable raw gas or rich endothermic mixture of fuel and air. The gas admitted by the tubes will be cracked endothermically to liberate carbon and hydrogen for contact with the work. These elements by virtue of their strong reducing tendency serve to reduce or prevent the formation of scale on the work, or, if desired, may be supplied in sufficient quantity to produce carburization of the work surface.
In order to prevent premature cracking of these raw gas additions and the consequent deposition of soot in the tubes, they are preferably provided with a cooling jacket through which cooling air may be circulated.
In bringing the Ness furnace up to heat, the air to fuel mixture is preferably readily combustible with a high flame temperature, as for instance, with natural gas, an air to fuel ratio of about 10 to 1. After the desired furnace temperature has been attained and before the work is placed in the work chamber, the air to fuel ratio is reduced either to the desired rich exothermic or endothermic range necessary to produce the carbon dioxide to carbon monoxide ratio in the work chamber required for the protection of the work. The burners are specially designed to preheat the gases within the burner.
U.S. Pat. No. 2,799,490, Two Stage Combustion Furnace, by Rusciano, copended with the Ness patent. Rusciano notes that, in order to produce the work protective atmosphere from the rich air/fuel mixture, it is necessary to supply external heat to the constituents to increase the reaction temperature. The normal temperature of these reactions are not sufficient to drive them to completion with the results that some solid would be formed. If the additional heat is not supplied the air to fuel ratio of the mixture must be increased and consequently the CO.sub.2 /CO and H.sub.2 O/H.sub.2 ratios will be increased to a point where they no longer represent non-scaling conditions in the furnace. If efforts are made to overcome this difficulty by increasing the fuel content of the mixture, lower reaction temperatures with increased soot formation is obtained.
In the Rusciano patent the furnace has a work heating chamber supplied with the plurality of burners to which a rich mixture of fuel and air is supplied for combustion directly in the work heating chamber. In order that the products of this combustion shall be non-scaling character it is necessary that the ratio of the fuel to air be such as to produce, upon completion of the thermal reactions, resulting products in which the sum of the CO.sub.2 /CO and H.sub.2 O/H.sub.2 ratio is equal to 1.0. With gaseous fuels this unity ratio summation is obtained with approximately 52% of the air that would be required for complete combustion of the fuel. With oil fuels the ratio may be increased up to about 58% depending upon the C/H.sub.2 ratio of the fuel.
The primary combustion of the air and the fuel is effected between radiant tubes and a wall of the furnace in a portion of the chamber away from the work whereby the reaction products receive radiant heat both from the tubes and the hot walls of the furnace and by conduction in passing in contact with such walls and tubes in transit to the work. The primary air/fuel mixture which has a deficiency of air, in the order of 50%, is supplied to the work chambers by a series of burners disposed in the sidewalls of the furnace directly above the radiant tubes. The arched roof of the furnace chamber together with the radiant tube surfaces act as high temperature catalysts to promote the primary reactions as the gases are scrubbed thereover. Burners are set in opposite walls enhancing the agitation the gases receive in contacting these areas. Additionally, the furnace chamber may be provided with a number of built up thin refractory arch sections extending between radiant tubes which are parallel to the roof of the heating chamber. These sections absorb heat from the tubes and serve as additional hot refractory for contact with the reacting products. They also serve to channel a reaction product more intimately into contact with the tubes and thereby enhance the absorption of heat from the tubes during the passage of the gaseous products to the work. In addition, the arches act as radiant surfaces for heating of the work thus assisting in maintaining a more uniform heat distribution of the work.
It is desireable to directly heat treat articles in a sealed furnace treatment chamber without the necessity of having a cumbersome chamber geometry and supplemental heat supply necessary in the prior art.